The present invention generally relates to a cryogen shot blast system and, more specifically, to a cryogen shot blast system having a recirculation system for particulate media.
Molded articles often have thin pieces of unwanted material extending therefrom called "flash" which must be removed from the articles for the articles to reach their desired final configuration. Removing flash from articles formed from flexible materials such as rubber, plastics, and the like, is difficult in view of the soft, elastic nature of the flexible materials. While various types of mechanical trimming operations have been proposed for use in removing unwanted flash, these methods have proven to be not economical in a number of applications.
In order to simplify and reduce the cost of flash removal, various attempts have been made for freezing or otherwise cooling molded articles to embrittle the thin sectioned flash, whereafter one or a combination of mechanical processes have been utilized to break-off, trim, or otherwise remove the frozen or embrittled flash. Some of these methods have utilized a two-stage process wherein the work pieces to be deflashed are cooled in a first stage to effect flash embrittlement, whereafter the work pieces are vibrated, tumbled, or otherwise mechanically treated in a second stage to break away or otherwise remove the embrittled flash. One method is to use a cryogen material, such as liquid nitrogen, to effect embrittlement of the work piece flash. As utilized herein, the term "cryogen" will be understood to refer broadly to substances which are fluids and are at temperatures of about -60 F. and below.
Two-stage processes of this type are undesirable from several viewpoints. They are time consuming to carry out because cooling the work pieces and removing their flash comprise separate steps that are carried out sequentially rather than concurrently. Inasmuch as the work pieces are cooled only once and will not be cooled again at other stages of the flash removal procedure, adequate time must be devoted at the outset to providing a thorough cooling of the work pieces to assure that they are refrigerated to an extent that their flash will remain embrittled throughout the remainder of the flash removal process. Sometimes the extensive degree of refrigeration which is required results in the generation of undesirable stresses and/or the formation of cracks or other types of structural defects in the work pieces. An equally troubling drawback of the two-stage processes is that, if there is a relatively large quantity of flash to be removed, the work pieces may not remain adequately embrittled during the entire time required for deflashing. Where such is the case, the work pieces are not properly deflashed.
These drawbacks have been overcome by shot-blast deflashing machinery which operate with a single flash embrittling and removing stage. For example, see U.S. Pat. Nos. 4,519,812, 4,598,501, 4,646,484, 4,648,214, and 5,676,588, the disclosures of which are expressly incorporated herein by reference in their entirety. While such machinery performs in an exemplary manner, there is a never ending desire to decrease the required time and/or cost of a deflashing operation. Accordingly, there is a need in the art for an improved cryogen shot-blast deflashing system.